Survey

Survey Description

The Fourstar Galaxy Evolution Survey (ZFOURGE) was conducted using the FourStar imager (Persson et al. 2013) on the 6.5m Magellan Baade telescope at Las Campanas Observatory. The use of medium-band filters in the near-IR (van Dokkum et al. 2009) allows for accurate sampling at wavelengths that bracket the Balmer break of galaxies leading to more well-constrained photometric redshifts at 1 < z < 4 than with broadband filters alone. In conjunction with existing optical through mid-IR photometry, this dataset provides a comprehensive sampling of the 0.3 – 8 micron spectral energy distribution of galaxies.

ZFOURGE is composed of three 11′ x 11′ pointings with coverage in the CDFS (Giacconi et al. 2002), COSMOS (Capak et al. (2007) and UDS (Lawrence et al. 2007). The 5sigma depth in a circular aperture of D=0.6” in the Ks band is 26.2-26.5 in the CDFS, COSMOS and UDS fields respectively at a typical seeing of ~0.4″. For more information regarding ZFOURGE consult Straatman et al. (2016).

5 sigma AB magnitude depths (CDFS, COSMOS, UDS; D=0.6” apertures)

J1

25.6

26.0

25.6

J2

25.5

26.0

25.9

J3

25.5

25.7

25.6

Hs

24.9

25.1

25.1

Hl

25.0

24.9

25.2

Ks

26.2-26.5

25.5

25.7

Ks depths correspond to a co-addition of the Fourstar imaging with public Ks-band imaging.

[Field]

[Coordinates]

[Total Survey Area]

[Number of Objects]

[Kband FWHM]

CDFS

[03 32 27, -27 45 52]

[128 sq. arcmin]

[30911]

[0.46”]

COSMOS

[10 00 31, +02 17 03]

[135 sq. arcmin]

[20786]

[0.47”]

UDS

[02 17 15, -05 11 53]

[189 sq. arcmin]

[22093]

[0.44”]

Comparison of secure spectroscopic redshifts from the literature to photometric redshifts across all three pointings in ZFOURGE. Top: of the 1,526 sources compared, we find a NMAD scatter in ∆z/(1 +zspec) of 0.10 with about 3% of sources at ∆z/(1 + zspec) > 0.15. Bottom: the residual between the spectroscopic and photometric redshifts, divided by 1+zspec. The red solid, dashed and dotted lines indicate, respectively, ∆z/(1 + zspec) = 0 ± 0, ± 0.05, and ± 0.15.

Completeness corrected number counts for each field observed with ZFOURGE. Sources with < 80% completeness are shown with open symbols. We recover at least 80% down to very deep Ks-band magnitudes of 25.4 − 25.9 and 50% down to 25.6 − 26.2. For comparison, the Ks-band completeness corrected number counts of similar galaxy surveys are indicated in grayscale.

Three different redshift quality tests, σz from testing against spectroscopic samples, σz,pairs from a redshift pair test (Quadri et al., 2010) for sources with Ks < 23.5 AB mag, and the photometric redshift uncertainty from the p(z) from EAZY, σz,EAZY . These correspond well at all redshifts, indicating excellent redshift accuracies for ZFOURGE: 1 − 2%.

The benefit of using medium bandwidth filters in the near-IR on photometric redshift accuracy. σz, derived from a redshift pair test, is clearly smaller if the near-IR medium band filters are used (compared to near-IR broadband), especially at 1.5 < z < 2.5.